Abstract Similar to skeletal muscle myofibers, cardiomyocytes in the heart appear to be particularly susceptible to membrane instability and rupture with the slightest perturbation, in part because of their contractile status that produces ongoing mechanical deformation. Mutations in genes that disrupt or weaken the membrane anchoring proteins of the dystrophin-glycoprotein complex (DGC) or the integrin adhesion network causes a wide range of muscular dystrophies that also cause cardiomyopathy. We have recently shown that the thromobospondin gene family (Thbs1- 5) plays a critical role in membrane stability through both effects on the ER stress response and secretory pathways, as well as controlling the integrin complexes present on the sarcolemma. In our previous cycle of funding we showed that overexpression of select Thbs proteins in the heart or skeletal muscle can have remarkable effects on sarcolemma stability in heart and skeletal muscle cells, while KO mice for the same Thbs genes have reciprocal effects that complement exactly the overexpression effect. Thus, in this renewal application we will attempt to uncover the molecular mechanisms whereby the Thbs proteins serve to regulate sarcolemmal stability during ER stress and healing, in part, through coding for select integrin heterodimers on the cell surface of cardiomyocytes. We propose the unifying hypothesis that the Thbs proteins are stress-induced ER resident factors that facilitate healing after injury or with stress by serving as intra-vesicular chaperones for sarcolemmal delivery of integrins through the secretory pathway. To address this hypothesis we propose 3 specific aims. Aim #1 will compare Thbs3 versus Thbs4 in regulating cardiac sarcolemma stability and disease with doxorubicin induced cardiomyopathy and muscular dystrophy (models of membrane instability). Aim #2 we will attempt to rescue the enhanced cardiac disease phenotype and sarcolemmal instability of Thbs3 TG and Thbs4-/- mice by integrin overexpression in vivo. Aim #3 we will examine the molecular mechanisms whereby Thbs3 reduces membrane stability and Thbs4 increases it through integrin processing within the cell. Defining the molecular mechanisms in play and how selective integrin sorting is achieved by the Thbs proteins during stress stimulation and healing will suggest novel treatment approaches for MI injury and post-ischemic heart failure.